The present invention relates to fuel injection controllers and fuel injection control methods that supply fuel to internal combustion engines, and more particularly, to fuel injection controllers and fuel injection control methods that change fuel injection modes in accordance with the operating conditions of engines.
In a typical automotive engine, fuel is injected into an intake passage and mixed homogeneously with air that passes through the intake passage. The homogeneous air fuel mixture is then sent to combustion chambers defined in the engine. In each combustion chamber, the air fuel mixture is ignited by a spark plug. This burns the mixture and produces drive force.
The combustion of the air fuel mixture in such homogeneous state is normally referred to as homogeneous charge combustion. In an engine that performs homogeneous charge combustion, a throttle valve is located in the intake passage to adjust the amount of air fuel mixture drawn into the combustion chambers and thus control the engine torque.
However, in engines that perform homogeneous combustion, the throttling action of the throttle valve decreases the pressure in the intake passage. This increases energy loss due to pumping (pumping loss) when the air fuel mixture is drawn into the combustion chambers from the intake passage and thus decreases the efficiency of the engine.
Stratified charge combustion solves this problem. In stratified charge combustion, fuel is injected directly into each combustion chamber. This delivers a rich, highly combustible air fuel mixture to the vicinity of the spark plug. Ignition of the rich air fuel mixture burns the surrounding lean air fuel mixture. In an engine that performs stratified charge combustion, the engine torque is basically controlled by adjusting the amount of fuel injected toward the vicinity of the spark plug. Accordingly, the throttling by the throttle valve becomes unnecessary. Thus, pumping loss is reduced and the efficiency of the angina is improved. Furthermore, in an engine that performs stratified charge combustion, the overall air fuel mixture is usually lean. This improves fuel efficiency.
Japanese Unexamined Patent Publication No. 7-103050 describes an engine that performs stratified charge combustion and homogeneous combustion in accordance with the state of the engine. In this engine, a first type of fuel injector (direct injector) injects fuel directly into the combustion chamber of each engine cylinder. A second type of fuel injector (indirect injector) injects fuel into the intake passage. Each direct fuel injector is connected to a fuel distribution pipe. Fuel is pressurized and forced through the distribution pipe from a fuel tank by a high pressure pump, which is driven by the engine. The fuel delivered through the distribution pipe is directly injected into each combustion chamber by the associated direct fuel injectors.
The indirect fuel injector is connected to another fuel distribution pipe. Fuel is pressurized and forced through the distribution pipe from the fuel tank by a low pressure pump. The fuel delivered through the distribution pipe is injected into the intake passage.
Stratified charge combustion is performed when the engine speed and the depression degree of the acceleration pedal are both small. Fuel is injected from each fuel injector of the first type when the associated cylinder is in the late stage of the compression stroke. Homogeneous charge combustion is performed when either the engine speed or the depression degree of the acceleration pedal becomes great. Fuel is injected from the indirect fuel injector during the intake stroke of each cylinder. In this manner, the engine shifts combustion modes between stratified charge combustion and homogeneous charge combustion in accordance with the operating conditions of the engine.
When performing stratified charge combustion, fuel must be injected into each commotion chamber when the associated cylinder is in the late stage of the compression stroke. Thus, the fuel injection pressure of each direct fuel injector, or the fuel pressure in the fuel distribution pipe to which the direct fuel injectors are connected, must be maintained at a high pressure. Accordingly, if the fuel pressure in the distribution pipe is not within a predetermined range due to an abnormality in the high pressure pump or other reasons, the required amount of fuel may not be injected from each direct fuel injector.
In the engine described in the above publication, this problem is solved by stopping the injection of fuel from the direct fuel injectors. When an abnormality occurs in the high pressure pump, the pressure in the distribution pipe, to which the direct fuel injectors are connected, falls below an acceptable level. If an unacceptably low injection pressure is detected, it is determined that the required amount of fuel cannot be injected from the direct fuel injectors. In this case, the injection of fuel from the direct fuel injectors is stopped, and the indirect fuel injector is employed. Accordingly, stable operation of the engine is continued by changing the fuel injection mode if an abnormality occurs in the high pressure pump.
When the engine is started, the ignition of the fuel is difficult. Thus, combustion tends to be unstable. Accordingly, if fuel is injected directly into each engine cylinder from the associated direct fuel injector when starting the engine, it is preferable that the injection pressure in the direct fuel injector be high. High injection pressure results in the injection of vaporized fuel and enhances the ignition of the fuel. This shortens the length of time required to start the engine and improves efficiency when starting the engine.
However, the amount of fuel discharged from the high pressure pump is normally low when starting the engine. Therefore, it is difficult to increase the fuel pressure in the distribution pipe to a point at which the fuel can be vaporized in a satisfactory manner. Furthermore, when the engine is started, the temperature of the engine is normally low. Thus, the heat of the engine cannot be used to vaporize the fuel. As a result, the fuel may not be sufficiently vaporized even if the fuel pressure in the fuel distribution pipe is high enough to inject the required amount of fuel from the direct fuel injectors. This may lower the starting efficiency of the engine.
In the engine described in the above publication, the fuel injection mode is changed when the injection of the required amount of fuel is hindered due to a decrease in the fuel injection pressure of the direct fuel injectors. However, as long as the fuel injection pressure in each direct fuel injector is higher than a pressure value that enables the injection of the required amount of fuel, fuel is injected from the direct fuel injectors during the compression stroke, even when the engine is being started. Therefore, when the engine is started, fuel may not be vaporized sufficiently even if the fuel pressure in the direct fuel injectors is high enough to inject the required amount of fuel. This may lower the starting efficiency of the engine. Thus, in the prior art, the problem of inefficiency during engine starting has not sufficiently been dealt with.